Just having joined this list I am not yet aware of all of Nick's
arguments (hey Nick, glad to meet you at SVP!), however I would like to
share some of the findings of my thesis work on, appropriately enough,
diplodocid caudal vertebrae. Kent Stevens (you all may remember him from
the computerized cervical vertebrae talk) and I have talked of modeling
the tails of all Morrison Formation sauropod genera sometime in the near
future (using the reams of data that didn't get into my thesis). What I
can say at the moment is I went into my thesis project hoping to dispell any
notions of habitual tripodality. Though I acknowledged that it was
probably imperative they rise up to mate, I long disdained the "3 leg
feeding" concept. The results of my analysis are quite interesting.
One sees in Diplodoucs a shift in prezygapophysial angle of
articulation, from dorsally to medially facing articular surfaces. This
change occurs rather rapidly (within the first 12 caudal vertebrae).
Additionally, if the animal hiked itself up in the air, the tail
touches down at the transition from "standard" to "diplodocid" (the
double beam type) chevrons. Correlating rather nicely with this shift is
a change in caudal rib shape (especially for Diplodocus) AND neural spine
angle! Clearly some functional signal is being displayed here. Another
interesting side light, the most proximal caudal vertebrae often have
subcircular shaped centra (though I must hasten to point out that not all
Diplodocus proximal caudals have this shape). Also, some specimens of
Diplodocus have tall peduncles in their caudal vertebrae, others very
short. At first I believed I could use these characters to sort by
species, but then I discovered that at the Dry Mesa Dinosaur Quarry and
at Dinosaur National Monument one can see all of these characters from
one quarry. I am now of the notion that some of these characters
represent sexual dimorphism, though which goes to what sex is currently
beyond my ability to prove (though a new Diplodocus specimen from Dry
Mesa may answer many of these questions {I hope!}). Only Diplodocus has
a bifurcate neural spine in its proximal caudal vertebrae that amounts to
any depth. This cleft occurs as far back as caudal 9, with depths often
exceeding 60 mm! The medial walls of this bifurcation are very rugose
(for that matter the entire pre- and postspinal lamina of Diplodocus is
large and rugose), more evidence for the attachment of some pretty
serious musculature and/or ligaments. The caudal centra of Diplodocus are
procoelous through caudal 12 (typically farther), some being markedly
so!
The gist of the above is that, after showing these characters and
their various incarnations to engineers (who better?), it became apparent
that in at least some individuals these characters could be used to argue
for a tripodal stance, especially if the first 12 or so caudal vertebrae
acted as a "third leg". Diplodocus cervical vertebrae possessed a
wonderful range of ventral flexure (accourding to Kent Steven and Mike
Parrish's 3-d modeling of Diplodocus cervical vertebrae), which would be
useful in tripodal feeding.
Other interesting anomalies- vestigial hyposphene/hypantrum
complexes can be seen in seemingly all Morrison Formation diplodocids
(none farther than caudal 3, however, except in the case of Supersaurus,
where it is present (very pronouncedly I must add) through at least
caudal 7). Laminae are actually quite useful for generic identification
(though one must be careful as a few are quite variable).
Looking at other diplodocids produced different results.
Apatosaurus has very "petite" caudal vertebral neural spines that are
extremely compressed laterally (beyond caudal 3), the centra are
heart-shaped (through caudal 7) and the caudal ribs lose their transverse
("wing-like") expansion by caudal 4. Not at all what one would hope to see
in a constantly tripodal beast. Apatosaurus caudal centra are apneumatic
(as opposed to Diplodocus, whose centra are very pneumatic), shorter than
those of Diplodocus, and Apatosaurus chevrons have less pronounced rami.
The caudal dorsal vertebrae of Apatosaurus lack bifurcations by
dorsal 6 or so, while in Diplodocus the bifurcation is present through
dorsal 9 (both possess 10 dorsals). Both of these animals have MASSIVE
pelvi (with the edge going to Apatosaurus).
Picking and choosing just a few characters to argue for
tripodality is dangerous for diplodocids, as none possess the "ideal"
tripodal suite of characters, yet all have at least some amount of signal
for tripodality. As my studies continue I will continue to explore this
problem (learning beam theory, what attaches where, etc.), but it will
be some years before I accumulate enough data to back up one argument
over another.
I hope this message generates some interest and that it was
not too long (what can I say, I finally found a "captive audience" for my
thesis material!). Feel free to email me off-list with any comments or
questions.
Brian Curtice
Department of Anatomy
SUNY Stony Brook
Bcurtice@ic.sunysb.edu